Piston and process of making same



Patentedoct. 22 1929 UNITED STATES QPATENT' OFFICE GEORGE DONALD WEL'I'Y, F CLEVELAND, OHIO, ASSIGNOB TO ALUMINUM COMPANY or AMERICA, vnnm O1 PITTSBURGH, PENNSYLVANIA, A. GOBPOBA TJ ION OF PENNSYL- PISTON AND PROCESS OF MAKING SAME Io Drawing.

In replacing cast iron or similar metal parts by aluminum base alloy castings the service to which the castings will be subjected in handling and in use is often so severe that the castings must be suitably cast or treated, or both, to enable such castings to give satisfaction. For example, it has been found that aluminum base alloy pistons for internal combustion engines are better ableto meet the service requirement when cast in a permanent or quick cooling mold as contrasted with a sand, or slow cooling, mold.. The' frictional wear of these permanent mold cast pistons is less, the hardness and tensile strength are greater, and the elastic limit is higher than in the sand cast pistons.

Prior to my invention, aluminum base alloy castings, including permanent mold castings,

have had their physical properties improved by being subjected toa heat treatment involving heatingthe casting to a temperature of about 950 F., then quenching it and finally ageing it at a temperature between about 200 F. and about 300 F. This method of heat treatment has given good results but is diilicult to carry out commercially because of the high temperatures required.

I have discovered that the steps of heat ing to a temperature of 950 F. and quenching may be entirely eliminated when the castings are to be used for certain purposes and that by subjecting the unheat-treated castings merely to the ageing step, very desirable 4o properties may be obtained.

Castings of various sizes and shapes may be aged by my improved method without warpage, cracking or distortion which is aften met with in the higher temperature heat treatment above referred to.

Application filed August 2, 1924. Serial No. 729,683.

Copending application of Z. Jeffries and I R. S. Archer, Serial No. 7 07,868, filed April 21, 1924, is illustrative of the heat treatmentpracticed prior to my invention and involving the steps of heating the casting to high temperature, quenching and ageing. My invention and this application are to be distinguished from the method-of heat treatment disclosed therein in that I subject the previously unheat-treated and unquenched castings to the ageing step. I may cool the castlngs, as they come from the mold, in the open air to room temperature before being aged, or may age them as soon as they have cooled only to the ageing temperature.- My method applies to articles cast either in slow or rapid cooling molds but the more rapid the cooling the greater is the potentiality for hardening. It is possible, by sufliciently slow cooling, to destroy practically all hardening possibilities. However, the rates of cooling as ordinarily encountered in commercial casting practice now and heretofore leave some of the hardening possibilities. I

My method of heat treatment may be easily practiced since it does not necessitate the use of high temperature. It has resulted in marked improvement in physical properties in sand cast as well as permanent mold cast articles. As a resultof my process, the hardness has been increased to a point where the casting for an internil combustion engine.

Internal combustion engine pistons are called upon to withstand frictional wear against the cylinder and Wrist pin, and the hammering of the rings in the ring grooves. They should have a sufiiciently high impactvalue, that is, toughness, as shown by high resistance to breakage or cracking by impact or shock, substantially to eliminate breakage in machining, handling and in use in an engine. When the skirt is slit lengthwise, as is now common practice, it should be able to resist forces ten ding to bend the slit portions and to resist to a high degree permanent deformation in such portions.

y follows:

An internal combustion engine piston'composed of an aluminum base alloy containing about 10% of copper, about 0.25% of magnesium, about 1.25% of iron and such amounts of other impurities, including silicon, as .ordinarily occur in commercial aluminum, when hardness being in the thinner sections; and

cast in a sand mold, had a Brinell hardness of between and 80, depending on the thickness of the sections of the casting, the greater an impact value of about foot pounds to fracture the standard notched impact test bar used with a single blow Charpy machine having a pendulum weighing 5 kg. A similar casting made in a permanent mold had Brinell hardnesses varying from about 90 to about 110 with an average of about 105, depending on the thickness of the sections of the castg; and an impact value, similarly measured, of about .75 foot pounds.

Sand cast pistons of the foregoing composition, after being cooled in the open air to room temperature, were subjected to my method of ageing by being heated-at atemperature of about 300 F. for about twelve hours and then cooled in air to room temperature, after which is was found that their hardness had been increased from about to 110 Brinell and the impact value was reduced to about .45 foot pounds; whilesimilar pistons cast in permanent molds, cooled in air and then heated for about six hours at 300 F. had their hardness increased to between about to 120 Brinell with an average of about 115 and had their impact value reduced to about I .50 foot pounds.

The foregoing temperature and duration of heating are illustrative of obtainable results. I have found that the improvement in properties is notconfined to one particular temperature may be obtained, to a varying amount, with temperatures as low as about 250 F. and as high as about 600 F. The duration of treatment forthese various and intermediate temperatures varies widely from a few minutes at the higher temperatures to perhaps sixty hours at the lower temperatures. The hardening, for example, takes place slowly at the lower temperatures, the hardness method of ageing may be practiced as gradually increasing as the treatment is prolonged while the hardening time may be shortened to a few minutes at the high temperatures. In general the more rapidly the hardening is forced by the higher temperatures, the lower is the maximum hardness obtainable. When the maximum hardness is obtained, whether by prolonged treatment, even at low temperatures, or by short treatment at high temperatures, the impact value tends to diminish so that the best combination of high hardness consistent with high impact value is generally obtained at moderate temperatures and a moderate length of treatment time.

Sound castings of the foregoing compositions have their tensile strengths increased by my specified method as much as about 25% as compared with the strengths in the untreated condition. 7 It will be understood that an increase in hardness, in castings of these alloys, is accompanied by ,an elevation of the elastic limit of the metal. This property is evidenced by the increased resistance of the casting to permaneut distortion and the maintenance of the shape of the casting when in use even .when the walls thereof are cut apart and supported at only one end or side. This is particularly desirable in piston castings having skirts slit lengthwise, for the slit portions tend to maintain. contact with the cylinder walls when in use and thereby maintain undiminished the original amount of bearing area of the piston. I

Aluminum base alloys containing below 10% and down to about 4% of copper or other suitable alloying and hardening ingredients are similarly improved by the foregoing treatment, but as will be understood, the hardening effects of the treatment are not so great as with the higher content of copper or other suitable hardeners, the susceptibility of the alloys con:

taining copper to this treatment apparently depending in part on the amount of copper retained in solution in the alloys. The impact value will be greater with alloys having the lower content of copper or other suitable alloying and hardening ingredients.

With aluminum alloys containing from about 10% up to about 16% of copper. or similar alloying and hardening ingredients,

the hardening effects are somewhat greater than with below about 10% of copper, but are accompanied by correspondingly diminishing impact values, as will be readily understood.

' I do not wish to be understood as limiting my method to the alloys or castings above specially mentionedsince it may be employed with good results on aluminum base alloys containing copper in amounts higher than those specified and also on aluminum base alloys containing copper and additional added elements.

I find it preferable to practice my invention meters substantially free frem copper butcontain ing magnesium and silicon in quantities suffieient to be susceptible to the hardening effects of the described heat treatment. Other altoying ingredients such as nickel, chromium and manganese may be present in the fore going alloys but these metals do not respond to the heat treatment of my invention and are not referred to herein as hardeners althoughthese ingredients are often referred to in the art as hardeners. ,However, for the sake of simplicity and clearness, I prefer here to refer to those elements which do not respond to heat treatment merely as alloying ingredicuts. The term hardening ingredient or hardening composition appearing in the specification and appended claims refers to one or more of the alloyed metals which responds tothe heat treatment disclosed herein and which by so doing confers hardness on the treated alloy.

Although I have specified a preferred temperature of about 300 F, I may vary this temperature somewhat, say, about 25 either way without sacrificing the advantages of the preferred temperature.

, It will be understood that substantially uniform results may be obtained from my improved heat treatment because of the ease of maintaining unchanged the comparatively low temperatures employed.

l do not wish to be understood as limiting my improved treatment to a single tempera: ture for a given casting since I may subject the casting to one temperature for a certain period of time and to another temperature for another period. For example, I have treated a casting made in a permanent mold for aboutsix hours at a temperature of about 275 F. and then at a temperature of about 340 for about two hours with substantially the same combination ofproperties as when a temperature of about 300 F. was maintained for a period of about twelve hours on a similar casting.

Having thus disclosed my invention so that others may be able to practice the same, the scope of my invention and what I desire to secure by Letters Patent is defined in what is claimed. Y Y e What is claimed is: I

position as herein defined in an amount between about 3 percent and about 16 per cent which comprises heating the casting at a temperature of about 800 F" for from about two hours to about sixty hours.

2. The method of agein an unheat-treated chilled casting compose of an aluminum base alloy containing a [hardening composition as herein'defined in an amount between about 3 per cent and about 16 percent which comprises heating the casting at a tempera-- ture of between about 300 F. and 600 F. for not less than five minutes and not more than about sixty hours.

3. The improved method of treating a chill casting composed of an aluminum base alloy containing above'about 3 per cent and up to about'16 per cent of a hardening composition as herein defined including copper which comprises cooling the chill casting in air and then heating thecasting at a temperature of about 300 ll: for between about six hours and sixty hours.

t. The method of improving the physical properties of an ageable aluminum base alloy casting containing between about lper cent and up to about 16 per cent of ahardening composition as herein defined which comprises heating the unheat-treated casting at a temperature between about 250 F. and 600 ll. until a hardness is obtained which varies with the content of hardening composition present from about 80 points Brinell with about a 4 per cent content to about 155 points Brinell with about a 10 per cent content.

ln testimony whereof I hereunto afiix my signature this 29th day of July, 1924-.

/ GEORGE DONALD WELTY.

l. The method of ageing an ageable unheat-treated casting composed of an aluminum base alloy'containing a hardeningcom- 

